1. Field of the Invention
The present disclosure relates to proteins and genes responsible for plant defense against soybean cyst nematode (SCN) and the regulation of their expression in plant defense against SCN infection.
2. Description of the Related Art
Soybean cyst nematode (SCN; Heterodera glycines) is an obligate sedentary endoparasite, and is the most important pathogen for soybean. The average amount of soybean lost to SCN in the United States from 2006 to 2009 was 128.6 million bushels annually, which was valued at $1.286 billion annually (Koenning and Wrather, 2010). The successful invasion of soybean by SCN depends upon SCN's ability to establish a permanent feeding cell (also known as “syncytium”) within the roots of soybean. Infective juveniles penetrate into the root and migrate toward the vasculature. Near the vasculature, each juvenile selects a single cell, which is modified to allow for the incorporation of adjacent cells through progressive cell wall dissolution to form a multinucleate syncytium. The nematode derives nutrients from the syncytium for its growth and reproduction.
The primary management practice for this pathogen is by developing resistant soybean cultivars. Resistant cultivars have been developed by identifying SCN-resistant soybean germplasm from collections of plant introductions (PI) and incorporating the trait through conventional breeding programs. Although several sources of resistance have been identified, only a few PIs have been used in breeding programs due to undesirable traits associated with other resistance sources. The most predominant sources of resistance found in commercially available cultivars are derived from PI 88788, PI 54840 (Peking), and PI 437654. In all resistant cultivars, the infective juveniles are capable of penetrating into roots and can induce the formation of syncytia, but the syncytia become necrotic soon after establishment and the nematodes starve to death. Although necrosis is a common theme, the timing of necrosis and degeneration of syncytia vary among resistant cultivars, depending on the source of resistance (Acedo et al., 1984). For example in Peking, syncytial collapse is observed as early as 48 hours post infection (Mahalingam and Skorupska, 1996); whereas, the onset of the resistance response is much slower in PI 209332, with degeneration of the syncytia not occurring until 8-10 days post infection (“dpi”) (Acedo et al., 1984).
Despite the extensive histological studies documenting the cellular changes associated with degenerating syncytia in soybean (Endo, 1965; Riggs et al., 1973; Acedo et al., 1984), very little is known about the molecular mechanisms underlying this hypersensitive-like resistance response. Research conducted in the past decade has identified a number of quantitative trait loci (QTLs) associated with SCN resistance (reviewed in Concibido et al., 2004) in different PIs that serve as sources of resistance in breeding programs. Among these, two major QTLs are Rhg1 on soybean chromosome 18 (formerly linkage group G) and Rhg4 on chromosome 8 (formerly linkage group A2). Rhg1 exhibits incomplete dominance and contributes to a significant portion of SCN resistance in most PIs tested, including PI 88788, PI 90763, PI 209332, and Peking (Concibido et al., 2004). In addition, Rhg1 is effective against a broad spectrum of SCN populations. Rhg4 is dominant and is required for full resistance to certain SCN populations in some (e.g., Peking, PI 437654), but not all (e.g., PI 209332, PI 88788), resistant sources (Brucker et al., 2005).
Microarray analyses have been carried out to study this plant-nematode interaction. Initial studies used whole soybean roots infected with SCN to assess transcriptional changes during a compatible interaction (Khan et al., 2004; Alkharouf et al., 2006; Ithal et al., 2007a; Klink et al., 2007a). However, due to the specialized nature of the interaction and the location of syncytia well within the root, it is very difficult to draw meaningful conclusions using whole roots to understand this pathosystem.
Laser capture microdissection (LCM) of syncytial cells coupled with microarray analysis has been particularly useful in extending our understanding of the SCN-soybean interaction, as indicated by recently published studies (Klink et al., 2005; Ithal et al., 2007b). These studies have provided new insights into the underlying molecular events occurring during syncytium development. More recently, the same technology has been applied to study incompatible SCN-soybean interactions (Klink et al., 2007b; Klink et al., 2009; Klink et al., 2010). Two studies reported on a comparative microarray analysis of soybean genes induced in response to either a virulent or an avirulent SCN population on Peking (Klink et al., 2007b; Klink et al., 2009), demonstrating that soybean can differentiate between nematode populations prior to feeding cell establishment (Klink et al., 2007b). The same group also published a microarray study that examined the transcriptional changes occurring in syncytia induced by an avirulent SCN population on PI 88788 at three time points after infection (Klink et al., 2010).
There have been no reports of a direct comparative analysis of syncytia gene expression profiles using near-isogenic lines (NILs) to identify transcripts regulated by specific soybean resistance genes. NILs have several advantages over PIs for comparative analyses of plant gene expression between resistant and susceptible soybean in response to SCN. Theoretically, NILs can share up to 98% of their genome, differing only in a region encompassing a trait of interest (Li et al., 2004); thus, NILs are powerful tools to study the effects of specific gene loci with reduced genetic background effects. Consequently, the use of NILs for molecular studies is becoming more popular. For instance, NILs have been used in a microarray analysis of iron efficient and inefficient cultivars of soybean (O'Rourke et al., 2009) and a wheat leaf rust resistance gene Lr10 (Manickavelu et al., 2010). NILs have also been used recently to help identify the effects of the Arabidopsis gene FLC on seed germination (Chiang et al., 2009). Despite intensive cytological and molecular genetic studies, the genes responsible for SCN resistance have not been identified (Melito et al., 2010), and the mechanism for resistance on a molecular level has yet to be fully elucidated.